Differences in Photosynthetic Characteristics and Leaf Microstructure Between the Karst-Associated Narrow-Ranged Camellia longzhouensis and Its Widespread Congener Camellia gigantocarpa

To examine differences in resource use and functional traits between the narrow-ranged Camellia longzhouensis and its widespread congener Camellia gigantocarpa, we compared their light and CO2 responses, leaf anatomy, stomatal traits, photosynthetic pigments, and trait correlations under common nursery conditions. In both species, net photosynthetic rate increased rapidly with increasing irradiance and then gradually approached a plateau. The maximum net photosynthetic rate of C. gigantocarpa was significantly higher and was 4.10 times that of C. longzhouensis, whereas apparent quantum yield, light saturation point, light compensation point, and dark respiration rate did not differ significantly between the two species. As intercellular CO2 concentration increased, net photosynthetic rate increased and then gradually leveled off in both species. Initial carboxylation efficiency was significantly higher in C. gigantocarpa, whereas the other CO2-response parameters showed no significant differences between species. Leaf thickness and lower epidermal thickness were similar between species, but C. gigantocarpa had a thicker upper epidermis and palisade tissue and a higher palisade-to-spongy tissue thickness ratio, whereas C. longzhouensis had thicker spongy tissue and higher stomatal density. Stomatal length, width, and area did not differ significantly between species. C. gigantocarpa had significantly higher chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid contents, as well as a higher chlorophyll a-to-b ratio, whereas the ratio of carotenoids to total chlorophyll did not differ significantly between species. Trait correlations indicated coordinated variation among photosynthetic performance, leaf anatomy, stomatal characteristics, and photosynthetic pigments. Overall, the two species exhibited distinct combinations of leaf structural and photosynthetic functional traits under common nursery conditions, providing baseline ecophysiological information on their resource-use differences and for germplasm conservation of C. longzhouensis.